It was a pin that held a thumb on the arm, but maybe it was from a hydraulic cylinder originally, I'm not sure. Apparently there was a regulator missing and the pressure was too high, which is how it managed to shear through almost 2" of steel

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Did a small test drill this evening, it doesn't actually seem that hard, so it should be workable.
As much as I would enjoy seeing what a slug this size could do, I'm more interested in small launchers. If the low x prototype I'm working on is successful, this will likely turn into version two of a bolt action cartridge hybrid.